The present invention relates to a level control system operated by pressure media, and in particular, a level control system for a vehicle in which at least one air suspension bellow is provided for support of the vehicle frame above at least one vehicle axle, and wherein the level of the vehicle frame above the vehicle axle is determined by the prevailing pressure in the air suspension bellow.
A level control system of this type is disclosed, for example, in German patent application DE 195 46 730 A 1, which is incorporated herein by reference. In such known system, inclination of a vehicle caused by asymmetrical loading is compensated for only when leveling of the vehicle can be achieved while concurrent maintenance of a required minimum pressure level in the air suspension bellows via which the vehicle body is supported on the vehicle axles is possible. When leveling of the vehicle cannot be accomplished while the minimum pressure level is maintained, the inclined position of the vehicle body relative the vehicle axles is temporarily retained, and subsequently remains unchanged until the occurrence of a different load situation in which leveling permits maintenance of the minimum pressure level.
In the known system, a test is carried out to determine whether leveling is possible without reduction of pressure below the required minimum pressure level, in accordance with which a number of opposing control attempts are made in order to compensate for the deviation sensed by the control system. If the number of attempts do not exceed a predetermined number, maintenance of the minimum pressure level is ensured. The multiple opposing control attempts, however, require introduction and venting of air, resulting in an overall loss of air pressure affecting the air supply system.
In addition to vehicle inclinations caused by loading, inclinations of the vehicle body may result from uneven footing, such as for example, where one wheel of the vehicle axis rests atop the sidewalk.
The problem of undesirable air loss attendant the previously mentioned air admission and venting processes involved with a leveling of the vehicle in general is exaggerated in circumstances in which the vehicle inclination is the result of uneven footing as compared to a load-related inclination. All air losses which are not absolutely necessary should advantageously be avoided, particularly in level control systems where the air supply system is fed by a compressor driven by an electric motor and which provides a limited air supply.
The above stated problem is further aggravated by the fact that when the vehicle has gone from the condition of uneven footing, which resulted in the inclined position, to a footing which is again even, the air suspension bellow pressures which changed in the course of compensation while in the inclined position must again be reversed, and this again involves air loss.
In the event of vehicle inclinations caused by uneven footing, the procedure in accordance with known systems used for inclinations caused by loads cannot, therefore, be effectively applied, since application of such would exaggerate the problem of undesirable air loss.
It is therefore the object of the present invention to provide a level control system operated by pressure media of the type mentioned above in which no unnecessary air losses occur, particularly when a vehicle inclination is caused by uneven footing.